A mobile radio communications system is generally divided into a number of cells, where a cell serves a particular geographical area. Within a cell, there is provided a radio base station via which a user equipment, present in the cell, may communicate. A user equipment may move from cell to cell, and, if the user equipment is engaged in communication upon crossing of a cell border, a handover should be performed from the radio base station in the cell exited, referred to as the source cell, to a base transceiver station in the entered cell, referred to as the target cell. A handover between cells may also be initiated for a user equipment that does not move, for example if the radio base station with which the user equipment is communicating experiences a high traffic load and the transmission circumstances in the neighbouring cell are more favourable.
In a system based on the Long Term Evolution (LTE) standard, a so called Random Access (RA) preamble is transmitted at handover by a user equipment on the Random Access CHannel (RACH) to identify to the target cell that the user equipment requires access to the target cell in order to complete the handover, as described for example in the 3rd Generation Partnership Project Technical Specification (3GPP TS) 36.300 vs. 8.0.0. A RA preamble is a pre-defined signalling sequence. RA preambles are also used for example for random access of non-active user equipments which access a cell and for which time alignment is required. There are a limited number of RA preambles available in a cell, and a RA preamble can only be successfully used by one user equipment in the cell at a time.
Hence, there is a risk of a shortage of RA preambles. If there is no RA preamble available for a user equipment for which handover should be performed, the handover will be delayed, with an increasing risk of handover failure. Similarly, if there is no RA preamble available for a non-active user equipment attempting to access a cell, the access attempt will be delayed. Not only may the delays result in reduced quality of service, but may even result in radio link failure.